The Universal Mobile Telecommunication System (UMTS), also referred to as the third generation (3G) system or the wideband code division multiplexing access (WCDMA) system, is designed to succeed GSM. UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system. In the UTRAN architecture, user equipment (UE) 150 is wirelessly connected to a NodeB 130, as illustrated in FIG. 1.
High-Speed Downlink Packet Access (HSPDA) is an evolution of UTRAN bringing further enhancements to the provisioning of packet-data services both in terms of system and end-user performance. The downlink packet-data enhancements of HSDPA are complemented with Enhanced Uplink (EUL), also known as High-Speed Uplink Packet Access (HSUPA). EUL provides improvements in the uplink capabilities and performance in terms of higher data rates, reduced latency, and improved system capacity, and is therefore a natural complement to HSDPA. HSDPA and EUL are often jointly referred to as High-Speed Packet Access (HSPA). The evolution of UTRAN has been strongly focused on increased data rates and reduced Round Trip Times (RTT), to support the use of delay sensitive packet-data services. RTT is defined as the time it takes for a packet to get to a remote place and back again. In order to allow for reduced RTT and increased data rates, the Transmission Time Interval (TTI) is shortened. TTI is defined as the duration of data transmission where coding and interleaving is performed.
In a UTRAN, the dedicated transport channel is called Dedicate Channel (DCH). The DCH carries all the information to/from a specific UE from/to higher layers including the data for the actual service and higher layer control information. The DCH is mapped onto two physical channels: The Dedicated Physical Data Channel (DPDCH) carrying higher layer information (which includes the user data), and the Dedicated Physical Control Channel (DPCCH) carrying the physical layer control information.
In a UTRAN with HSPA, the EUL enhancements are implemented through a new dedicated transport channel: the Enhanced Dedicated Channel (E-DCH). A short TTI of 2 ms is supported by the E-DCH to allow for reduced RTT (as discussed above). An alternative TTI length of 10 ms is also supported, and the network can configure the appropriate TTI value. Simultaneous transmission of E-DCH and DCH is possible. The uplink data and control channels used for E-DCH support are illustrated in FIG. 2. The E-DCH is mapped to a set of uplink channelization codes known as E-DCH Dedicated Physical Data Channels (E-DPDCH) 200. In order for the NodeB to demodulate and decode the data transmission on the E-DPDCH 200, control signaling is needed on the uplink, and the E-DCH Dedicated Physical Control Channel (E-DPCCH) 201 is used for this purpose. The DPCCH 202 carries the pilot signal and transmission power control commands. There is also an uplink channel for the HSDPA related control signaling, called HSDPA Dedicated Physical Control Channel (HS-DPCCH) 203.
Although a short TTI is generally beneficial for upper layer protocols and applications, there is a downside as well: The reliability of the transmitted data (and thus the coverage) decreases with a shortened TTI, as a shortened TTI means reduced energy per information bit. One solution to this problem is to increase the transmission power and thus increase the energy per information bit. This might be possible on the downlink, but in the uplink and especially in the case of a coverage limited scenario, the UE is in general already transmitting close to its maximum transmission power.
Another solution to the problem is to reduce the number of information bits and thus to get more energy per information bit. Though reducing the number of data channel information bits (within a TTI or subframe) may increase the coverage, this approach has the disadvantage of increased control overhead, i.e. an unbalanced relation between the control channel information and the data channel information.
Yet another solution to the problem of decreased reliability or coverage is to rely on retransmissions on lower layers, typically Hybrid Automatic Repeat Request HARQ retransmissions. This solution is used in UTRAN HSPA systems.
HARQ is a combination of forward error-correcting (FEC) coding and Automatic Repeat Request (ARQ). In FEC coding, redundancy is introduced in the transmitted signal. Parity bits are added to the information bits prior to the transmission, and the parity bits are computed from the information bits using a method given by the coding structure used. In an ARQ scheme, the receiver uses an error-detecting code to detect when the received packet is in error and a retransmission is requested in that case. If no error is detected, a positive acknowledgement (ACK) is sent to the transmitter, and if an error is detected, a negative acknowledgement (NAK) is sent. HARQ thus uses FEC codes to correct a subset of all errors and relies on error detection with retransmission for handling the rest of the errors.
Taking the example of HARQ retransmissions in uplink in a UTRAN with HSPA, the NodeB receives a subframe, comprising both the physical control channels (e.g. DPCCH, HS-DPCCH and E-DPCCH) and the physical data channels (E-DPDCH), in an initial transmission from the UE. Based on the E-DPCCH, the NodeB will try to demodulate and decode the E-DPDCH. If there are non correctable errors in the physical data channel information, the information cannot be decoded and the NodeB will ask for a retransmission of the subframe by sending a NAK to the UE. The entire subframe is then retransmitted by the UE.
One disadvantage of the retransmission solution is that the physical control channel information is retransmitted together with the physical data channel information in every retransmission, although the control channel information has already been received in the initial retransmission. A part of the scarce uplink transmission power resource is thus used for redundant control data.